The present invention relates to a simple and cost-effective method for producing iron(III) orthophosphate-carbon composites (FOP/C) with a high electrical conductivity, iron(III) orthophosphate-carbon composites produced according to the method, as well as their use for the production of LiFePO4 cathode materials for Li-ion batteries.
Rechargeable Li-ion batteries are widely used energy storage means, in particular in the mobile electronics sector, since the Li-ion battery is characterised by a high energy density and can supply a high rated voltage, so that the Li-ion battery with a comparable performance is significantly smaller and lighter than conventional batteries. Spinels such as LiCoO2, LiNiO2, LiNi1-xCoxO2 and LiMnnO4 have proved to be suitable as cathode materials. In order to increase the safety of the Li-ion batteries, especially with regard to a thermal overloading during operation, LiFePO4 was developed as a cathode material. This material is characterised by a good performance, high specific capacity and also high thermal stability in operation. Iron orthophosphate is a starting material for the production of LiFePO4 cathode material for Li-ion batteries.
High demands in terms of purity are placed on the cathode material of Li-ion batteries, since any contamination that may involve undesirable redox reactions during operation (charging and discharging) has a deleterious effect on the performance of the battery. The nature and concentration of the possible contaminations basically depends on the quality of the raw materials used for the production of the cathode material and their production processes per se. In the production process of the cathode material measures can be adopted for the subsequent reduction of impurities, which however is generally associated with an increase in production costs. It is therefore desirable to use starting materials and raw materials that are as pure as possible for the production of the cathode material. Apart from the purity of the starting materials, their structure and morphology also have a significant influence on the quality of the cathode material produced therefrom.
DE 10 2009 001 204 A1 describes the production of crystalline iron(III) orthophosphate (FOP) in the form of phosphosiderite crystallites (metastrengite II crystallites) with a particular morphology and purity. On account of the particular purity and the novel material properties this iron(III) orthophosphate (FOP) is particularly suitable as a starting material for the production of lithium-iron phosphate (LiFePO4; LFP) for lithium ion batteries, for example according to the methods described in US 2010/0065787 A1.
Pure lithium-iron phosphate (LFP) has a poor electrical conductivity, which is why it can only be used to a limited extent in its pure form as a cathode material. Various approaches have therefore been developed in order to improve the electrical conductivity of lithium-iron phosphate.
U.S. Pat. No. 6,855,273 B2 and US 2010/0065787 A1 describe the production of a carbon coating on the LFP particles, in which a synthesised LFP or a mixture of precursor compounds, inter alia FOP, is mixed with organic materials, generally oligopolymers or polymers, and is then heated for several hours at temperatures around 700° to 800° C. in order to effect a carbonisation of the organic component on the surface of the LFP particles. If no graphitisation is thereby achieved, this can have a negative effect on the electrical conductivity of the cathode material, since only graphitic structures ensure a high electrical conductivity. The process parameters of this thermal process have to be strictly controlled, which is complicated. Also, the carbon precursor compounds required for the formation of the coating have to be chosen so as to match the process exactly. A further disadvantage is that the carbon precursor compounds have to be added in excess in relation to the carbon fraction remaining in the end product, since a part of the precursor compounds is lost in the form of thermal decomposition products. The exact adjustment and reproduction of the carbon and graphite content is complicated on account of the process.
Another disadvantage of this method is that in the thermal process a temperature of at least 650° C. must be achieved in order to carbonise and graphitise an organic carbon precursor compound. At such high temperatures it is virtually impossible to prevent a pronounced particle growth and a caking of the calcination material. However, this in particular should be avoided in the production of LFP, in order to keep the diffusion paths for the Li ions short.
US 2009/0311597 A1 describes the doping of LFP with different transition metals or transition metal compounds in order to produce cathode materials with acceptable electrical conductivities. The doping additives can in this connection be distributed homogeneously in the sense of a mixed crystal in the material or can be present as a separate crystalline phase in addition to the LFP. The doping with transmission metals or also with lanthanoid metals involves high costs for these doping additives per se and in addition requires very complicated and costly methods in order to achieve a distribution and doping that raises the conductivity. Thus, for example, US 2009/0311597 A1 discloses very high calcination temperatures of 800° C. and long calcination times of up to 96 hours, which economically is a serious disadvantage.
US 2009/0152512 A1 describes a material similar to that of US 2009/0311597 A1, though in this case exclusively nanocrystals of metal oxides are discussed, which should be present as separate phases in a cathode material matrix in order thereby to raise the electrical conductivity of the resulting material.
US 2003/0064287 A1 discloses that iron phosphates were intimately mixed with acetylene black in a ratio of 5:1 by means of a dry ball mill for 15 to 120 min (generally 90 min) in order to test the iron phosphates for activity in electrochemical cells. This ratio corresponds to a carbon content of about 17%. In this connection the particle sizes of amorphous, nano-scale iron phosphates should not alter. A crystalline iron phosphate was however comminuted from a mean particle size of about 5 μm to 500 nm. In addition the document assumes that an improved effectiveness of a carbon coating of the iron phosphate particles is achieved by increasing the mixing time. However, it was not demonstrated that a carbon coating of the iron phosphate particles was actually achieved, but was simply assumed.
The addition of extremely fine carbon particles, such as acetylene black, superP (Timcal) or Ketjen Black (Akzo Nobel), or also carbon nanotubes with their extremely special properties, appears relatively simple compared to many other described methods. These special carbons must however in turn be produced by special methods, which restricts their market availability and also makes these materials significantly more expensive compared to say conventional graphites.
A high carbon addition to the active material (cathode material) of a battery in order to achieve the necessary electrical conductivity is not economical, since a battery produced in this way would have to lose potential storage capacity at the expense of the carbon component. It is therefore desirable to achieve a sufficient electrical conductivity with at the same time as low a carbon content as possible. Apart from this the processing of slurries of the cathode material becomes more difficult with increasing carbon content, as is described for example in EP 1 094 532 A1.